The present invention relates to a technique of recording and reproducing information on and from an optical disk representing a recording medium, and more particularly, to an information recording and reproducing technique which can prevent S/N degradation in a reproduction system due to cross talk with the reproduction system caused by circuit operation in a recording system when the information (inclusive of address information recorded on the recording medium in advance) is read out of the recording medium.
In the field of products concerning the optical disk such as CD, DVD and the like, an increase in capacity of the optical disk and an increase in speed of data transfer in an optical disk apparatus have recently been desired more and more. Also, with the capacity of the optical disk increased, mark and space (corresponding to 1, 0 of information) to be formed on the optical disk are required to be finer and formation of such fine mark and space is required in the optical disk apparatus.
In order to form accurate and fine mark and space, one mark recording waveform is required to have a multi-pulse form in a drive current waveform for a semiconductor laser during recording and a pulse position or pulse width at the mark start end and a pulse position or pulse width at the mark termination end must be controlled adaptively in accordance with a mark length of its own and an adjoining space length. For example, in a DVD-RAM, the aforementioned adaptive control of the pulse position or pulse width is required to be performed in T/16 to T/32 steps (T being a minimum unit for determining the mark and space lengths and corresponding to the period of a so-called channel clock chCLK).
Further, the drive current waveform is also needed to be quaternary in contrast to the conventional binary and so has become complicated. In addition, with the data transfer speed increased, the frequency of the aforementioned drive current waveform has become higher.
As the capacity and data transfer speed increase in this manner, current at multiple-valued levels supplied to the semiconductor laser must be switched at a high speed. To assure such a high-speed current switching characteristic as above (rise characteristic: Tr characteristic and fall characteristic: Tf characteristic of the drive current), it is preferable that the semiconductor laser drive circuit be disposed in the proximity of the semiconductor laser.
To meet this requirement, a conventional semiconductor laser drive circuit is so constructed as to have a plurality of current sources at least one of which is externally selected to drive the semiconductor laser. Therefore, as the drive current waveform has multiple valued levels, the number of control signal lines for selecting the current source increases. Further, when the semiconductor laser drive circuit is carried on an optical pickup, control signals are supplied through a flexible cable, facing a difficulty that because of dullness of the control signal waveforms and the difference (skew) in delay amount between control signals, the accurate drive current waveform cannot be obtained.
As an optical disk apparatus for solving the difficulty as above, the present applicant has proposed an optical disk apparatus described in Japanese Patent Application No. 10-206083 filed on Jul. 22, 1998, which was published on Oct. 15, 1999, as Japanese Patent Application Laid-Open No. 11-283249. 
A semiconductor laser drive circuit shown in Japanese Patent Application No. 10-206083 is comprised of drive waveform information memory means for storing one or more information of drive waveform which drives a semiconductor laser in compliance with a binary recording signal (for example, NRZ signal) to be recorded on a recording medium, drive waveform decoding means for decoding the drive waveform on the basis of the information stored in the drive waveform information memory means, means for generating an address for selecting the drive waveform information of the drive waveform information memory means on the basis of the binary recording signal, control means for storing the externally supplied drive waveform information in the drive waveform information memory means, and n-multiplying means (so-called PLL) for n-multiplying a clock signal CLK supplied through a flexible cable to deliver a channel clock signal chCLK and supplying the channel clock signal as operation clock signal for the address generation means and the drive waveform decoding means.
The drive waveform decoding means generates timing signals for N equal division (for example, 16 or 32 equal division in the DVD-RAM) of the period T of the chCLK by using a delay line on the basis of the channel clock signal chCLK delivered out of the n-multiplying means, thus permitting the aforementioned control of the pulse width or pulse position.
With this construction, when the drive waveform information is stored in the drive waveform information memory means in advance through the control means, the drive current waveform is generated during recording by supplying the binary recording signal (NRZ), the clock signal CLK and a signal WRgate for control of the recording or reproducing operation mode to the semiconductor laser drive circuit carried on the optical pickup through the flexible cable, so that the semiconductor laser can be driven nearby and the difficulty of the conventional example that the drive current waveform is distorted owing to the dullness or skew of the control signals on the flexible cable can be eliminated to thereby assure desired Tf and Tf characteristics.
In case the multiplying number n of the n-frequency means is set to 4, the frequency of the clock signal CLK equals xc2xc of the chCLK and as compared to the conventional example (the current source selecting signal having the same frequency as chCLK must be supplied), the frequency of the control signals on the flexible cable can be reduced to xc2xc as compared to the conventional example and the EMI generated from the flexible cable can be reduced. To add, in the DVD-RAM, the frequency of the NRZ signal (repetition of 3T length mark and 3T length space gives a maximum frequency) is ⅙ of the channel clock signal chCLK.
As the capacity of the optical disk increases (densifying), however, the level of the reproduction signal for reading information from the recording medium decreases and for the sake of assuring reliability of reproduction data, the influence such as cross talk from the recording circuit system to the reproduction circuit system must be more decreased. The optical pickup carries photodetectors for detecting a reflection light beam from the optical disk and I-V amplifiers for converting output currents of the photodetectors to voltages, and outputs of the amplifiers are supplied to a read channel LSI and the like through the flexible cable. Accordingly, the influence of the crosstalk from the recording circuit system to the reproduction circuit system must be reduced on the flexible cable to prevent degradation of the reproduction S/N.
Especially, when a sector is defined as a unit of recording (storage of 2 Kbyte user data) as shown in a DVD-RAM (2.6 GByte) illustrated at (1) in FIG. 4 and physical address information of each sector in the form of pits is recorded on the head of each sector (this area is called PID), the PID must be reproduced sector by sector to confirm the address and then data to be recorded on a user data area of each sector must be recorded. In other words, even during recording of data, not only recording of data is carried out but also recording and reproduction of data is carried out repetitively.
When, as in the case of the semiconductor laser drive circuit constructed as above, the supply of clock signal CLK to the semiconductor laser drive circuit by way of signal lines such as the flexible cable (hereinafter simply referred to as xe2x80x9cflexible cablexe2x80x9d) is kept even during PID reproduction as shown at (4) in FIG. 4 by taking stability of the frequency of the clock signal chCLK (output signal of the n-multiplying means) during user data recording following the PID into consideration, EMI of the clock signal CLK on the flexible cable (unwanted radiation emitted by the clock signal on the flexible cable) runs in a read signal(at (5) in FIG. 4) on the same flexible cable or on a flexible cable of the separately provided photoelectric conversion circuit to cause crosstalk, giving rise to a problem that the S/N of the reproduction signal is degraded.
An object of the present invention is to decrease leakage of the EMI of the clock signal necessary for recording of data to the reproduction signal and degradation of the S/N of the reproduction signal due to crosstalk and to suppress an increase of the EMI level which causes the S/N degradation of the reproduction signal.
To accomplish the above object, according to the present invention, an optical disk apparatus comprises a semiconductor laser drive circuit having LD recording drive current waveform generating means for generating a recording drive current waveform for a semiconductor laser by using a clock signal chCLK as operation clock when recording information on a recording medium and reproduction drive current supply means for supplying a reproduction drive current to the semiconductor laser when reading the information from the recording medium, recording clock supply means for supplying the clock signal chCLK to the semiconductor laser drive circuit, recording/reproduction mode control means for controlling recording/reproducing operation of the semiconductor laser drive circuit, and clock stop means for stopping the supply of the clock signal chCLK from the recording clock supply means to the semiconductor laser drive circuit when the output of the recording/reproduction mode control means designates reproducing operation.
Also, to accomplish the above object, according to the present invention, an optical disk apparatus comprises a semiconductor laser drive circuit having multiplying means for providing a clock signal chCLK which is n times (n being positive integer) the frequency of a clock signal CLK, LD recording drive current waveform generating means for generating a recording drive current waveform for a semiconductor laser by using the clock signal chCLK as operation clock when recording information on a recording medium and reproduction drive current supply means for supplying a reproduction drive current to the semiconductor laser when reading the information from the recording medium, recording clock supply means for supplying the clock signal CLK to the semiconductor laser drive circuit, recording/reproduction mode control means for controlling recording/reproducing operation of the semiconductor laser drive circuit, and clock stop means for stopping the supply of the clock signal CLK from the recording clock supply means to the semiconductor laser drive circuit when the output of the recording/reproduction mode control means designates reproducing operation.